fixed bug #929: close async service before the main thread finishes (#931)

half-sync-half-async/src/main/java/com/iluwatar/halfsynchalfasync/App.java

@@ -94,6 +94,8 @@ public class App {
     service.execute(new ArithmeticSumTask(500));
     service.execute(new ArithmeticSumTask(2000));
     service.execute(new ArithmeticSumTask(1));
+
+    service.close();
   }
 
   /**

half-sync-half-async/src/main/java/com/iluwatar/halfsynchalfasync/AsynchronousService.java

@@ -22,6 +22,9 @@
  */
 package com.iluwatar.halfsynchalfasync;
 
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
 import java.util.concurrent.BlockingQueue;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.ExecutorService;
@@ -38,6 +41,7 @@ import java.util.concurrent.TimeUnit;
  */
 public class AsynchronousService {
 
+  private static final Logger LOGGER = LoggerFactory.getLogger(AsynchronousService.class);
   /*
    * This represents the queuing layer as well as synchronous layer of the pattern. The thread pool
    * contains worker threads which execute the tasks in blocking/synchronous manner. Long running
@@ -95,4 +99,16 @@ public class AsynchronousService {
       }
     });
   }
+
+  /**
+   * Stops the pool of workers. This is a blocking call to wait for all tasks to be completed.
+   */
+  public void close() {
+    service.shutdown();
+    try {
+      service.awaitTermination(10, TimeUnit.SECONDS);
+    } catch (InterruptedException ie) {
+      LOGGER.error("Error waiting for executor service shutdown!");
+    }
+  }
 }